Ultrasonic transducers, which transmit and receive ultrasonic waves, are used for apparatuses for diagnosing tumors in human bodies etc., apparatuses for performing nondestructive tests of structures etc.
As the ultrasonic transducers, those utilizing vibration of piezoelectric substances have so far been used. However, with progress of the MEMS technology in recent years, capacitive micromachined ultrasonic transducers (CMUTs) comprising a vibration part formed on a silicon substrate have been developed, and researches are actively conducted aiming at practical use thereof.
For example, U.S. Pat. No. 6,320,239 B2 (Patent document 1) discloses a single CMUT and a CMUT array.
U.S. Pat. No. 6,571,445 B2 (Patent document 2) and U.S. Pat. No. 6,562,650 B2 (Patent document 3) disclose techniques for forming CMUT in an upper layer of a signal processing circuit formed on a silicon substrate.
Further, U.S. Patent Published Application Nos. 2005/0228285 A1 (Patent document 4) and 2007/0264732 A1 (Patent document 5) disclose CMUTs having a structure comprising a projection protruding into a hollow part.